1. Field of the Disclosure
This disclosure relates to a plasma-based method and apparatus for treating a substrate. In particular, the disclosure relates to a plasma-based method and apparatus for generating a neutral beam of particles for performing an anisotropic and mono-energetic neutral beam activating chemical processing of a substrate by applying a non-ambipolar electron plasma in a low-pressure environment.
2. Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
During semiconductor processing, plasma is often utilized to assist etch processes by facilitating the anisotropic removal of material along fine lines or within vias (or contacts) patterned on a semiconductor substrate. Examples of such plasma assisted etching include Reactive Ion Etching (RIE), which is in essence an ion activated chemical etching process.
However, although RIE has been in use for decades, its maturity is accompanied by several issues including: (a) broad ion energy distribution (IED), (b) various charging-induced side effects; and (c) feature-shape loading effects, that is, micro loading. One approach to alleviate these problems is to utilize neutral beam processing as described in commonly owned or assigned, U.S. Pat. Pub. 2009/0236314, herein incorporated by reference.
A true neutral beam process takes place essentially without any neutral thermal species participating as the chemical reactant, additive, and/or etchant. The chemical process, such as an etching process, at the substrate is activated by the kinetic energy of the incident (directionally energetic) neutral species and the incident (directionally energetic and reactive) neutral species also serve as the reactants or etchants.
One natural consequence of neutral beam processing is the absence of micro loading since the process does not involve the effect of flux-angle variation associated with the thermal species (which serve as the etchants in RIE). However, an adverse consequence of the lack of micro loading is the achievement of an etch efficiency of unity, that is, the maximum etching yield is unity, or one incident neutral nominally prompts only one etching reaction. Conversely, the abundant thermal neutral species (the etchant) in RIE can all participate in the etching of the film, with the activation by one energetic incident ion. Kinetic energy activated (thermal neutral species) chemical etching can therefore achieve an etch efficiency of 10, 100 and even 1000, while being forced to live with micro loading.
Current neutral beams may use, for example, a turbo-molecular pump (TMP) utilizing a rather unreasonable 10,000 liters/second (l/s) flow rate placed upon delicate substrates, for example, 300 mm wafer substrates.
FIG. 1 is a schematic view of a conventional neutral beam (NB) source 10 where a neutralizer grid 20 is at ground. FIG. 1 describes the pumping difficulty of the conventional neutral beam (NB) source. In other words, where the TMP or turbo 28 is high, for example, 10,000 liters/second (l/s), when a thin wafer substrate 26, for example 300 mm wafer substrate, is exposed to the same, the wafer substrate may fail or break. In FIG. 1, NB source 10 may include a first plasma chamber 16 for forming a first plasma 18 at a first plasma potential (VP,1) at approximately 10 millitorr (mTorr), and a second plasma chamber 22 for forming a second plasma 24 at a second potential (VP,2) at approximately 1×10−4 to 5×10−5 Torr and the second potential is greater than the first plasma potential. The first plasma 18 is formed by coupling power, such as radio frequency (RF) or microwaves (μ-wave) at 12, to an ionizable gas, for example, argon (Ar) gas, in the first plasma chamber 16 via a gas injector inlet 14, while the second plasma 24 is formed using electron flux passing through the neutralizer grid 20 from the first plasma 18.
The first plasma chamber 16 comprises a plasma generation system 12 configured to ignite and heat the first plasma 18. The first plasma 18 may be heated by any conventional plasma generation system including, but not limited to, an inductively coupled plasma (ICP) source, a transformer coupled plasma (TCP) source, a capacitively coupled plasma (CCP) source, an electron cyclotron resonance (ECR) plasma source, a helicon wave plasma source, a surface wave plasma source, a surface wave plasma source having a slotted plane antenna, and the like. Although the first plasma 18 may be heated by any plasma source, it is desired that the first plasma 18 is heated by a method that produces a reduced or minimum fluctuation in its plasma potential (VP,1). For example, an ICP source is a practical technique that produces a reduced or minimum (VP,1) fluctuation (see U.S. Pat. Pub. 2009/0236314).
FIG. 2 is a graphical plot showing a potential diagram and geometry 30 of an accelerator surface 32 and a grounded neutralizer grid top surface 34 using a conventional neutral beam (NB) source of a mono energetic type of FIG. 1. In this type, the accelerator surface 32 of a mono-energetic NB has to be direct current (DC) powered (+VA). In FIG. 2, a plasma bulk 36 has a plasma potential (VP) or boundary-driven plasma potential, as driven up by the positively biased DC accelerator surface 32, where VP˜VA. It should be noted that the accelerator surface 32 has a surface area substantially greater than the surface area of the neutralizer grid top surface 34. Further, the potential diagram and geometry 30 also shows a sheath S comprising a classical pre-sheath SA governing the ion Bohm velocity and the initial ion flux, a sheath edge 38, and an electron-free region 40 or cathode fall SB, where the total sheath S=SA+SB.
It should be noted that the DC biased accelerator surface 32 comprises a relatively large area in contact with the plasma bulk 36. The larger the area at DC ground, the lower the first plasma potential. For example, the surface area of the conductive surface for the DC biased accelerator surface 32 in contact with the plasma bulk 36 may be greater than any other surface area in contact with the plasma bulk 36.
Additionally, for example, the surface area of the conductive surface for the DC biased accelerator surface 32 in contact with the plasma bulk 36 may be greater than the total sum area of all other conductive surfaces that are in contact with the plasma bulk 36.
Alternatively, as an example, the conductive surface for the DC biased accelerator surface 32 in contact with the plasma bulk 36 may be the only conductive surface that is in contact with the plasma bulk 36. The DC biased accelerator surface 32 may offer the lowest impedance path to ground.
While many attempts have been made to cure these shortcomings, that is, etch efficiency, micro loading, charge damage, TMP flow rates, and/or tradeoffs between these parameters, they still remain and the etch community continues to explore novel, practical solutions to this problem.